Container production in nursery and greenhouse operations using soilless media involves inputs of fertilizers, growth regulators, insecticides, and fungicides. Repeated excessive irrigation leads to leaching and loss of nutrients and chemicals in runoff. The presence of nutrients in runoff and concerns of their impact on surface and groundwater quality has undergone increasing interest and scrutiny from the public, environmental groups, governmental agencies, and elected officials. Since its enactment, the U.S. Environmental Protection Agency (EPA) has enforced provisions of the Clean Water Act (1972) related to point-source pollution. In 1999, the EPA began enforcing nonpoint source pollution controls specified in section 303(d) of the Clean Water Act, which mandates that all states implement a Total Maximum Daily Load (TMDL) program for all watersheds and bodies of water (U.S. EPA, 2000). A TMDL is the maximum amount of pollutant that a water body can receive from point and nonpoint sources and still maintain its designated use and value (e.g., drinking water, fish and wildlife habitat, recreation, and so on). The Clean Water Act (U.S. EPA, 1994) lists nitrogen (N) and phosphorus (P) as potential pollutants of impaired water bodies. Offsite movement of nitrate–nitrogen (NO3 −) and soluble reactive phosphate (H2PO4 −, HPO4 2−, and PO4 3−) from nursery and greenhouse operations may lead to excessive algal and aquatic plant growth in surface waters, resulting in accelerated eutrophication. In general, freshwater systems are P-limited and more prone to P inputs, whereas N often limits primary production in estuarine and marine environments (Carpenter et al., 1998).
The maximum contaminant level for NO3 − in drinking water is 10 mg·L−1 (National Academy of Sciences, 1977). No federal limits on P contamination in freshwater have been established as a result of variations in size, hydrology, and depth of rivers and lakes and regional differences in P impacts. However, the U.S. EPA recommends that total P not exceed 0.05 mg·L−1 in any streams discharging into lakes or reservoirs and 0.10 mg·L−1 in streams or other flowing waters that do not (U.S. EPA, 1986).
Fertigation runoff in greenhouse crop production can contain 100 mg·L−1 NO3-N (Wood et al., 1999). In nursery crop production, nursery runoff NO3-N concentrations range from 0.1 to 135 mg·L−1 (Alexander, 1993; Taylor et al., 2006; Yeager et al., 1993) and P levels from 0.01 to 20 mg·L−1 (Alexander, 1993; Headley et al., 2001; James, 1995; Taylor et al., 2006). These cited N and P runoff ranges could be higher or lower in other nursery and greenhouse crop production systems.
Recently TMDLs of nutrients in agricultural runoff were adopted by environmental regulatory agencies in every state (Yeager, 2006). This follows a trend in which state governments have been passing more stringent laws and regulations assessing and regulating nonpoint sources of pollutants beyond the scope of the provisions of the Clean Water Act.
Constructed wetlands (CWs) have been promoted as an inexpensive, low-technology approach to comply with increasingly stringent environmental regulations regarding the discharge of nonpoint source pollutants in greenhouse and nursery production (Arnold et al., 1999; Berghage et al., 1999). Surface-flow (SF) and subsurface flow (SSF) CWs are two commonly used wetland designs to treat agricultural wastewater (Berghage et al., 1999; Scholz and Lee, 2005). A SF CW resembles a shallow (0.2 to 0.8 m) freshwater marsh and generally requires a large land area for wastewater treatment (Kadlec and Knight, 1996). To remediate nursery and greenhouse wastewater, surface area can be reduced with a concomitant increase in depth (≈1.25 to 1.5 m), which promotes anaerobic conditions that facilitate denitrification.
Alternatively, greenhouse and nursery operations constrained by limited production space and expensive land can use a SSF CW, which consists of a lined or impermeable basin filled with a coarse medium, typically gravel, and wetland plants (Kadlec and Knight, 1996). Wastewater flows horizontally or vertically below the surface of the media to prevent exposure to humans or wildlife. SSF CWs can be operated in continuous-flow or batch-load treatment modes with varying hydraulic residence times (Burgoon et al., 1995).
Nitrogen removal from SSF CWs is accomplished primarily by denitrification and plant uptake (Vymazal, 2007). Inorganic or organic P, which has no valency changes during its biotic assimilation or microbial decomposition, is mainly removed through microbial and plant uptake (Vymazal, 2007). Roots and rhizomes support rhizospheric microorganisms by providing colonizing sites exuding carbohydrates, sugars, amino acids, enzymes, and many other compounds (Rovira, 1969) and oxidizing the rhizosphere (Wießner et al., 2002), which fosters microbial activity.
One of the many factors that control the efficiency of nutrient and bacterial removal in wetlands is vegetation type (Guntenspergen et al., 1989). Wetland plants have species-specific efficiencies regarding their abilities to aerate water, grow within the constraints of the wetland environment, and remove nutrients and heavy metals (Maschinski et al., 1999). Previously studied aquatic emergent plants for CWs include reed canarygrass (Phalaris arundinacea L.), common reed [Phragmites australis (Cav.) Trin. Ex Steud.], reed mannagrass [Glyceria maxima (Hartman) Holmb.], softstem bulrush [Schoenoplectus tabernaemontani (C. C. Gmel.) Palla], yellow flag (Iris pseudacorus L.), and cattail (Typha spp. L.) (Ansola et al., 1995; Hunter et al., 2001; Wolverton et al., 1983). They have not been widely used because of their potential invasiveness. Additionally, their high rates of biomass production necessitate periodic harvesting to prevent the seasonal export of nutrients, particularly P, through vegetative decomposition (Hunter et al., 2001).
In this study, we investigated a cost-effective approach suggested by Adler et al. (2003): “One way to reduce water treatment costs is to produce a product of value concomitant with treatment of the water.” Instead of traditional wetland plants, commercially available aquatic garden plants can be used in a production/remediation system that could generate revenue. Few studies have examined the ability of aquatic garden plants to thrive in SSF CWs and recover nursery runoff rates of N and P (Arnold et al., 1999, 2003; Holt et. al, 1999).
In an earlier study, we investigated the potential of seven aquatic garden plants to assimilate N and P in a laboratory-scale, gravel-based SSF CW system (Polomski et al., 2007). Louisiana Iris hybrid ‘Full Eclipse’ exhibited the highest N recovery rate, whereas similar P recovery rates were observed in Canna × generalis Bailey (pro sp.) ‘Bengal Tiger,’ Canna × generalis Bailey (pro sp.) ‘Yellow King Humbert,’ Iris ‘Full Eclipse,’ Peltandra virginica (L.) Schott, and Pontederia cordata L. ‘Singapore Pink’ (Polomski et al., 2007). Our objective was to investigate five additional commercially available aquatic herbaceous emergent garden plants—three upright and two creeping—for their ability to thrive and recover N and P in a laboratory-scale wetland system that approximated a SSF CW.
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